The present invention relates generally to the monitoring of filtration systems. More specifically, the present invention relates to the monitoring of reverse osmosis systems.
A vast array of businesses depend on filtration technology. Food and beverage industries, especially the beer and wine industry, utilize filtration systems everyday. Likewise, filtration systems are vital in the chemical processing, paper and refining industries. Among other things, filtering liquids is used to accomplish one or more of the following: water purification, concentration, purification of product solution or suspension, and removal of outside contaminants (sterilization).
The fine filtration of liquids is commonly referred to as membrane technology. Membrane processing is subdivided into three technologies: microfiltration, ultrafiltration and reverse osmosis. These technologies are used to separate suspended or dissolved materials from a solvent in applications ranging from contaminant removal for water purification to solute concentration in water treatment or processing applications. Cartwright, An Overview of Fine Filtration Technology, Pharmaceutical and Cosmetic Equipment, p. 39 (March, 1985)
Microfiltration involves removal of particulate material ranging in size from 0.1 to 10.0 microns. Ultrafiltration separates materials in the 0.001 to 0.1 micron range, whereas reverse osmosis is used for separations involving materials less than 0.001 micron in size. Microfiltration is primarily used for removal of suspended or colloidal materials; ultrafiltration and reverse osmosis are used for the separation of dissolved material (solute). Id.
Since many industries require high purity water, more and more industrial plants are installing reverse osmosis systems. A significant operating cost factor of a reverse osmosis systems is the cost of the membranes themselves. If properly maintained, membranes can last for years before replacement becomes necessary. However, deposition of material on membrane surfaces may result in increased energy consumption, or membrane failure which can ultimately cause an unscheduled shutdown and significant replacement costs. Accordingly, an utmost concern for plants using a reverse osmosis system is a proper monitoring program.
Membranes are susceptible to a loss of performance as a result of accumulation of small particles, colloids, oil, microorganisms, and precipitated salts on their surfaces. Some of these deposits cause catastrophic membrane failure in a short period of time, while others affect membrane performance over longer periods of time. These deposits are known as scaling and fouling, or collectively as membrane deposition.
Premature failure of reverse osmosis membrane elements due to identified or unidentified membrane fouling substances costs thousands to millions of dollars each year. Membrane fouling has been cited as the single largest cause, if not the only cause, of permeate flux decline at normal operating pressures and temperatures in brackish water systems. Fouling, as used in this context, refers to the accumulation of a substance or substances on or in the membrane. Such fouling causes a reduction in water transfer per unit area of membrane (flux). Paul et al, Reverse Osmosis, Membrane Fouling--The Final Frontier?, Ultrapure Water, p. 25 (April, 1990).
In addition to membrane fouling, other sources are known that lead to membrane destruction. For instance, as stated above, scaling can ultimately lead to the destruction of a reverse osmosis membrane. Scaling refers to a coating which forms on the membrane due to the precipitation or crystallization of salt compounds or solids.
Typical steps taken to prevent scaling include lowering the pH of the waters to reduce scaling potential, and feeding antiscalant treatments to ensure any scale that may form will remain in a dispersed state. In addition, extensive pre-treatment systems are frequently used to remove particulate matter (media and cartridge filters), iron (green sand filters), and other potential foulants. Zeiher et al, Microbial Control, Biofouling of Reverse Osmosis Systems: Three Case Studies, Ultrapure Water, p. 50 (October, 1991).
A variety of chemical and mechanical pre-treatments have been utilized in an attempt to control fouling. Treatments relating to the prevention of film formation fall into two main categories: (1) removal of fouling bacteria from the feedwater, and (2) metabolic inactivation by chemical means.
Complete removal of fouling from the feedwater can be technically problematic and economically unfeasible. One method often used is filtration of the feedwater. Due to the small size of most bacteria (&lt;1 micron), pore sizes must be very small. This frequently results in rapid plugging of the filter by bacteria and colloids, forcing continual filter replacement.
Chemical addition may be the most effective method of feedwater pre-treatment. The application of the correct dosage at the correct frequency is essential in order to maintain a continuous active biocide residual on the membrane surface. Disinfection of the feedwater is most commonly achieved by the addition of chlorine or other oxidants. However, since composite membranes are degraded by oxidants, the oxidants must be removed before contacting the membrane. This, in turn, leaves the membrane surface vulnerable to microbial attack.
Moreover, another mechanical treatment method is ultraviolet sterilization. Although such ultraviolet sterilization can be effective, it does not provide an active residual, thereby permitting rapid regrowth of surviving bacteria. Accordingly, reverse osmosis systems are not infallible, and even the most extensively pre-treated water can cause fouling.
In an effort to prevent poor reverse osmosis performance, unscheduled down time, and the premature (and expensive) replacement of membrane elements, plants strive to set up an effective monitoring program. Probably the most common detection scheme relies on periodic monitoring of particular system parameters. To properly assess the performance of an entire bank of membrane elements, experts suggest that one must compare the current system performance to its start-up performance. In doing so, the three most useful performance parameters to track each day and evaluate carefully are: percent salt rejection, normalized permeate flow rate, and membrane bank differential pressure. These three performance parameters, in conjunction with the feedrate flow, approximate the extent of fouling, scaling, and membrane degregation--the three major causes of premature membrane element failure. Bukay, Membranes, The Basis of Monitoring Reverse Osmosis--Part 1, Ultrapure Water, pp. 58-59 (October, 1992).
On the other hand, while these parameters indicate the existence of a potential problem, they cannot provide accurate information regarding the source of the problem. For instance, when the system experiences a permeate flow loss, the technician is not able to determine whether fouling, scaling or some other source has caused the permeate flow loss. Moreover, periodic monitoring of these parameters only gives information about planktonic (suspended) organisms. Sessile (attached) organisms in water systems are of greater concern and often out number the planktonic counts by several orders of magnitude.
Other common detection schemes also do not provide effective results. Another detection scheme relies on scrapings taken from the housing during membrane cleaning and/or replacement. This technique often provides information too late, resulting in the need for membrane replacement. Moreover, often times the entire system must be shut down until the problem is identified, resulting in increased down time costs.
The other common detection scheme relies on destructive analysis. As the name implies, this technique requires the destruction of the membrane to allow for membranae surface micro-biological analysis. Naturally, this destructive technique is costly because the membranes cannot be returned to service. Resorting to these last two detection schemes can result in a loss of thousands of dollars in down time and membrane replacement costs.
Accordingly, while current monitoring programs facilitate the detection of membrane deposition, they fail to provide an accurate and early detection means. In fact, no published method for non-destructive observation of membrane surface fouling exists in the field. One expert has stated the following with respect to current monitoring programs:
Monitoring RO systems for fouling potential is a standard operating procedure at most facilities. That's the good news! The bad news is that there aren't any good monitoring methods that accurately predict either colloquial or biological fouling . . . It is obvious . . . that much more research is needed.
Paul et al, Reverse Osmosis, Membrane Fouling--The Final Frontier?, Ultrapure Water, pp. 30 & 33 (April, 1990).